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Cell structure
The cell (from cella, meaning "small room") is the basic structural, functional, and biological unit of all known . A cell is the smallest unit of . Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology. Cells consist of enclosed within a , which contains many s such as s and s. Organisms can be classified as (consisting of a single cell; including ) or (including s and s). The number of cells in plants and animals varies from species to species, it has been estimated that contain somewhere around 40 (4×1013) cells. , and is tall. The approximation is not exact, this study estimated that the number of cells was 3.72±0.81×1013.}} Most plant and animal cells are visible only under a , with dimensions between 10,000 and 1,000,000 . Cells emerged on Earth at least 3.5 billion years ago. Cell types Cells are of two types: , which contain a , and , which do not. All prokaryotes are , while eukaryotes can be either single-celled or . Prokaryotic cells Prokaryotes include and , two of the . Prokaryotic cells were the first form of on Earth, characterised by having vital es including . They are simpler and smaller than eukaryotic cells, and lack membrane-bound such as a . The of a prokaryotic cell consists of a single that is in direct contact with the . The nuclear region in the cytoplasm is called the . Most are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. A prokaryotic cell has three architectural regions: * Enclosing the cell is the – generally consisting of a covered by a which, for some bacteria, may be further covered by a third layer called a . Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as (bacteria) and (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting ( ) from due to a environment. Some eukaryotic cells ( s and cells) also have a cell wall. * Inside the cell is the that contains the (DNA), ribosomes and various sorts of inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry elements called s, which are usually circular. Linear bacterial plasmids have been identified in several species of bacteria, including members of the genus notably , which causes Lyme disease. Though not forming a nucleus, the is condensed in a . Plasmids encode additional genes, such as genes. * On the outside, and project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells. Eukaryotic cells components of a cell}} , , , s, , and are all . These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is : the presence of membrane-bound s (compartments) in which specific activities take place. Most important among these is a , an organelle that houses the cell's . This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include: * The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present. * The eukaryotic DNA is organized in one or more linear molecules, called s, which are associated with proteins. All chromosomal DNA is stored in the , separated from the cytoplasm by a membrane. Some eukaryotic organelles such as also contain some DNA. * Many eukaryotic cells are with primary cilia. Primary cilia play important roles in chemosensation, , and thermosensation. Each cilium may thus be "viewed as a sensory cellular that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." * Motile eukaryotes can move using motile or . Motile cells are absent in and . Eukaryotic flagella are more complex than those of prokaryotes. Subcellular components All cells, whether or , have a that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the . Inside the membrane, the takes up most of the cell's volume. All cells (except s which lack a cell nucleus and most organelles to accommodate maximum space for ) possess , the hereditary material of s, and , containing the information necessary to various s such as s, the cell's primary machinery. There are also other kinds of s in cells. This article lists these primary s, then briefly describes their function. Membrane The , or plasma membrane, is a that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a . This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a , which are (partly and partly ). Hence, the layer is called a , or sometimes a fluid mosaic membrane. Embedded within this membrane is a variety of molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is semi-permeable, and selectively permeable, in that it can either let a substance ( or ) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain proteins that allow cells to detect external signaling molecules such as s. Cytoskeleton The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during , the uptake of external materials by a cell, and , the separation of daughter cells after ; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of s, s and s. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called . The subunit of microtubules is a dimeric molecule called . Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include , , (lamins A, B and C), (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M). Genetic material Two different kinds of genetic material exist: (DNA) and (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is in its DNA sequence. RNA is used for information transport (e.g., ) and functions (e.g., RNA). (tRNA) molecules are used to add amino acids during protein . Prokaryotic genetic material is organized in a simple in the of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called s inside a discrete nucleus, usually with additional genetic material in some organelles like and (see ). A has genetic material contained in the (the ) and in the mitochondria (the ). In humans the nuclear genome is divided into 46 linear DNA molecules called s, including 22 pairs and a pair of . The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs. Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called . This can be transient, if the DNA is not inserted into the cell's , or stable, if it is. Certain es also insert their genetic material into the genome. Organelles Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the of the human body (such as the heart, lung, and kidney, with each organ performing a different function). Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound. There are several types of organelles in a cell. Some (such as the and ) are typically solitary, while others (such as , , and ) can be numerous (hundreds to thousands). The is the gelatinous fluid that fills the cell and surrounds the organelles. Eukaryotic * Cell nucleus: A cell's information center, the is the most conspicuous organelle found in a cell. It houses the cell's , and is the place where almost all replication and synthesis ( ) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the . The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, is , or copied into a special , called (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the . * Mitochondria and Chloroplasts: generate energy for the cell. are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. occurs in the cell mitochondria, which generate the cell's energy by , using to release energy stored in cellular nutrients (typically pertaining to ) to generate . Mitochondria multiply by , like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through . * Endoplasmic reticulum: The (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. The smooth ER plays a role in calcium sequestration and release. * Golgi apparatus: The primary function of the Golgi apparatus is to process and package the s such as s and s that are synthesized by the cell. * Lysosomes and Peroxisomes: s contain s (acid s). They digest excess or worn-out s, food particles, and engulfed es or . s have enzymes that rid the cell of toxic s. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. * Centrosome: the cytoskeleton organiser: The produces the of a cell – a key component of the . It directs the transport through the and the . Centrosomes are composed of two , which separate during and help in the formation of the . A single centrosome is present in the s. They are also found in some fungi and algae cells. * Vacuoles: s sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably , have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells. Eukaryotic and prokaryotic * Ribosomes: The is a large complex of and molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). Structures outside the cell membrane Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the . In order to assemble these structures, their components must be carried across the cell membrane by export processes. Cell wall Many types of prokaryotic and eukaryotic cells have a . The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of , fungi cell walls are made up of and bacteria cell walls are made up of . Prokaryotic Capsule A gelatinous is present in some bacteria outside the cell membrane and cell wall. The capsule may be as in , or as or as in . Capsules are not marked by normal staining protocols and can be detected by or ; which allows for higher contrast between the cells for observation. Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes. Fimbria A also known as a is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae, or pili are formed of a protein called ( ) and are responsible for attachment of bacteria to specific receptors of human cell ( ). There are special types of specific pili involved in . Cellular processes Replication Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in s (the growth of ) and to procreation ( ) in s. cells divide by , while cells usually undergo a process of nuclear division, called , followed by division of the cell, called . A cell may also undergo to produce haploid cells, usually four. cells serve as s in multicellular organisms, fusing to form new diploid cells. , or the process of duplicating a cell's genome, always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the . In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before . DNA replication does not occur when the cells divide the second time, in . Replication, like all cellular activities, requires specialized proteins for carrying out the job. Growth and metabolism Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: , in which the cell breaks down complex molecules to produce energy and , and , in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called such as . Once inside the cell, glucose is broken down to make adenosine triphosphate ( ), a molecule that possesses readily available energy, through two different pathways. Protein synthesis Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: and . Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called s located in the , where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule. Motility Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include and . In multicellular organisms, cells can move during processes such as wound healing, the immune response and . For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton. Multicellularity Cell specialization Multicellular organisms are s that consist of more than one cell, in contrast to s. In complex multicellular organisms, cells specialize into different s that are adapted to particular functions. In mammals, major cell types include , , s, s, s, , and others. Cell types differ both in appearance and function, yet are identical. Cells are able to be of the same but of different cell type due to the differential of the s they contain. Most distinct cell types arise from a single cell, called a , that into hundreds of different cell types during the course of . Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of s during ). Origin of multicellularity Multicellularity has evolved independently at least 25 times, including in some prokaryotes, like , , , or . However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants. It evolved repeatedly for plants ( ), once or twice for s, once for , and perhaps several times for , , and . Multicellularity may have evolved from of interdependent organisms, from , or from organisms in . The first evidence of multicellularity is from -like organisms that lived between 3 and 3.5 billion years ago. Other early fossils of multicellular organisms include the contested spiralis and the fossils of the black shales of the B Formation in . The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in using predation as the . Origins The origin of cells has to do with the , which began the on Earth. Origin of the first cell There are several theories about the origin of small molecules that led to life on the . They may have been carried to Earth on meteorites (see ), created at , or synthesized by lightning in a reducing atmosphere (see ). There is little experimental data defining what the first self-replicating forms were. is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see ), but some other entity with the potential to self-replicate could have preceded RNA, such as or . Cells emerged at least 3.5 billion years ago. The current belief is that these cells were s. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form. Origin of eukaryotic cells The eukaryotic cell seems to have evolved from a of prokaryotic cells. DNA-bearing organelles like the and the are descended from ancient symbiotic oxygen-breathing and , respectively, which were by an ancestral n prokaryote. There is still considerable debate about whether organelles like the predated the origin of , or vice versa: see the for the origin of eukaryotic cells. References Notes Category:Life